Wear resistant low-alloy valve steel

ABSTRACT

A wear resistant nickel-free austenitic stainless steel alloy, with relatively low alloy content, and particularly adapted for the manufacture of valves for internal combustion engines operating on low-lead or lead-free gasoline. The alloy consists essentially of, in weight percent, 0.25 to 0.55 carbon, 0.05 to 0.45 nitrogen, with carbon plus nitrogen within the range of 0.65 to 0.95, 10 to 14 manganese, 0.20 to 1.5 silicon, 15 to 20 chromium, 1 max. aluminum, up to 1.0 of elements from a group consisting of the strong carbide forming elements columbium, tantalum, titanium, zirconium and hafnium, 0.0008 to 0.005 boron and balance iron.

Dulis et a1.

1 1 .Ian. 21, 1975 WEAR RESISTANT LOW-ALLOY VALVE STEEL Inventors: Edward J. Dulis, Upper St. Clair Twp., Allegheny Co.;'William Stasko, Munhall, both of Pa.

Assignee: Crucible Inc., Pittsburgh, Pa.

Filed: Mar. 23, 1973 Appl. No.: 344,313

Field of Search 75/126 B, 126 D, 126 F,

References Cited UNITED STATES PATENTS Hsiao 7 5/126 B Jennings 75/126 B Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Arthur J. Steiner [57] ABSTRACT A wear resistant nickel-free austenitic stainless steel alloy, with relatively low alloy content, and particularly adapted for the manufacture of valves for internal combustion engines operating on low-lead or leadfree gasoline. The alloy consists essentially of, in weight percent, 0.25 to 0.55 carbon, 0.05 to 0.45 nitrogen, with carbon plus nitrogen within the range of 0.65 to 0.95, 10 to 14 manganese, 0.20 to 1,5 silicon, 15 to 20 chromium, 1 max. aluminum, up to 1.0 of elements from a group consisting of the strong carbide forming elements columbium, tantalum, titanium, zirconium and hafnium, 0.0008 to 0.005 boron and balance iron.

5 Claims, 8 Drawing Figures Pmmgmmzi m5 SHEEI 20F 6 FIG 2 2.0 SILICON CONTENT (WI.

0 v 4 w m m 0 RE 33 REG: MQQEWEE m mefifimumm.

SILICON CONTENT (WI.

O z I m 0 RE a3 59%: MQQEQEQ m @QREQEQ PATENIEUJANZI ms sum 8 or 6 Fla 8 C-HV TEST TEMPERATURE (F} WEAR RESISTANT LOW-ALLOY VALVE STEEL For use in the manufacture of valves for internal combustion engines it is customary to employ austenitic stainless steels. These steels, primarily because of their austenitic structure, exhibit high hardness and resistance to deformation at the high temperatures to which valves are subjected during service. In addition to hardness at elevated temperature it is necessary that alloys for this purpose exhibit resistance to oxidation. Also, the alloys must exhibit wear resistance to withstand repeated contact between the valve and valve seat. Prior to the introduction of lead-free gasolines it was also necessary that the valve exhibit resistance to lead oxide corrosion, thereby'limiting the silicon con-' tent and requiring for this purpose substantial additions of nickel. Nickel also contributed significantly to the formation of the required austenitic structure but, because it has been found in this invention that nickel significantly increases wear, this element is purposely limited in the alloy of this invention to not more than residual amounts.

It is accordingly a primary object of the present invention to provide an austenitic stainless steel for use in the manufacture of internal combustion engine valves to operate in a low-lead or lead-free environment, which alloy is nickel-free and yet is fully austenitic without the inclusion of other expensive alloying elementsin excessive amounts; the alloy, while having a low alloying element content, nevertheless exhibits the required hardness, resistance to deformation, wear resistance and resistance to excessive, detrimental oxidation.

This and other objects of the invention as well as a more complete understanding thereof may be obtained from the following description, specific examples and drawings, in which:

FIG. 1 is a graph showing the effect of carbon in combination with nitrogen on the hardness of alloys of the yp FIG. 2 is a graph showing the effect of silicon on the wear resistance of the alloy when tested in simulated valve application;

FIG. 3 is a graph showing the effect of silicon content on the wear resistance of the valve plus valve seat under the identical conditions of testing used in obtaining the results with respect to FIG. 2;

FIG. 4 is a. bar graph showing the total metal couple wear resistance for couples containing a nickel-free experimental alloy of this invention, and also two nickelbearing commercial alloys 21-4N and 21-2N, which are outside the scope of this invention;

FIG. 5 is a graph showing the relationship between tensile strength at elevated temperature, e.g. 2,100F, with the carbon plus nitrogen content of the alloy;

FIG. 6 is a graph showing the effect of chromium. with respect to the hot workability or ductility of the alloy;

FIG. 7 is a graph showing the effect on the alloy with respect to hot ductility of variations in the carbon plus nitrogen content, with changes in the chromium content; and

FIG. 8 is a graph showing the effect of silicon on the hot ductility of the alloy.

The above object maybe achieved with a nickel-free alloy in accordance with'the present invention having a composition consisting essentially, in weight percent, of carbon 0.25 to 0.55, nitrogen 0.05 to 0.45, total carbon plus nitrogen 0.65 to 0.95, manganese 1010 14, silicon 0.2 to 1.5, chromium 15 to 20, aluminum up to 1, up to 1.0 of one or more elements from the group consisting of columbium, tantalum, titanium, zirconium and hafnium, boron 0.0008 to 0.005 and balance iron. This composition, in accordance with the present invention, may be varied by providing for chromium within the range of'17 to 19 percent by weight as well as a strong carbide former such as columbium within the range of0.0l to 0.5, and preferably 0.01 to 0.3 percent by weight. Also, the alloy may be modified by having boron within the range of 0.0008 to 0.0015 percent by weight. Although the alloy is regarded as nickelfree, residual nickel, in accordance with typical melting practices for alloys of this type, may be present as an unintentional, residual addition in amounts up to about 0.3 percent by weight.

With the elimination or minimization of the use of lead in gasolines so that the automotive emission standards could be met, a significant wear problem is being encountered with current valve materials. Lead in gasoline serves as a high-temperature lubricant so that when this element no longer is added to gasoline, a se vere wear problem occurs in either the exhaust valve steel or the cast iron block. Thus, wear resistance and material compatibility to minimize total wear is an important new factor in the design of automotive valves and engine performance. In addition, other requirements previously significant to the performance of aatomotive exhaust valves, such as lead oxide corrosion resistance, are no longer primary factors in performance.

From the standpoint of oxidation resistance, as will be demonstrated by the specific examples provided hereinafter, this in accordance with the present invention is provided by a relatively low chromium content within the range of 15 to 20 and preferably 17 to 19 percent by weight in combination with either silicon alone or in combination with aluminum. Silicon for this purpose is present within the range of 0.20 to 1.5 percent and a portion thereof may be replaced by an amount of aluminum up to 1 percent by weight. The required austenitic structure is provided by the combination of carbon and nitrogen with these austenitepromoting elements being present in total within the range of 0.75 to 0.95 percent by weight. These elements, in combination with the high manganese content of 10 to 14 percent insure a fully austenitic structure. Further, strengthening is provided by the strengthening compounds of carbon, nitrogen and boron.

By way of demonstration with respect to the effect of the various alloying elements for obtaining the required properties for valve applications, 30-pound air induction melted heats of the alloys 1.116 to 1122 listed in Table were melted.

TABLE I COMPOSITIONS OF EXPERIMENTAL STEELS Material or Chemical Composition (Wt. 71) Heat No. Bar No. C Mn Si Cr Ni Al B N C+N TABLE I-Continued COMPOSITIONS OF EXPERIMENTAL STEELS Material or Chemical Composition (Wt. /t-) Heat No. Bar No. C Mn Si Cr Ni Al B N C+N U18 71-28 0.51 11.62 3.14 17.37 0.001 0.16 0.77 1.119 7l-29 0.37 11.78 1.20 17.87 0.12 0.001 0.35 0.72 1.120 71-30 0.44 11.89 1.20 17.64 0.65 0.001 0.20 0.64 1.121 71-31 0.43 11.99 17.64 1.56 0.001 0.17 0.60 H22 71-32 0.48 11.97 17.35 2.52 0.001 0.12 0.60 2l-2N 66-373 056 8.84 0.12 20.83 1.83 0.36 0.92 ill-4N 69-13 0.56 9.37 0.15 20.86 3.77 0.43 0.99

TABLE IA COMPOSITIONS OF EXPERIMENTAL STEELS Heat Chemical Composition (Wt. 7n)

No. 1 Bar No. C Mn Si Cr Ni Al B P Cb Ti N C+N 1136A 71-78 0.47 12.87 2.31 15.65 0.002 0.38 0.85 1137A 7 l-79 0.46 13.05 2.32 20.23 0.001 040 0.86 1138A 71-80 0.48 12.85 .32 18.46 0.001 0.15 0.41 0.89 1139A 71-81 0.47 13.00 2.28 18.40 0.001 0.12 0.42 0.89 1140A 71-82 0.46 13.09 2.25 18.46 0.001 0.02 0.41 0.87

With regard to the experimental compositions of Table I, 1116 to 1J22, the primary composition variables are silicon and aluminum, except for Heat Nos. 1117, M18 and 1119 which have increased carbon plus nitrogen content in an amount within the scope of the present invention. Ingots of the experimental compositions of Table l were soaked at 2,150F for three hours and subsequently forged to 34 in. sq. bars to obtain samples for mechanical property evaluation. Table II shows hardness values for 18 percent chromium, 12 percent manganese specimens of TABLE 1 containing silicon contents of0.2 percent, 1 percent, 2 percent and 3 percent silicon. The hardness values as listed in TABLE 11 show that in the solution treated and aged condition the hardness of all the specimens, except Bar No. 71-28 was lower than the minimum hardness of about 35 R desired for valve application. This hardness was attained in Bar 71-28 primarily because of its relatively high carbon plus nitrogen content. A plot of carbon plus nitrogen content versus hardness in the solution treated and aged condition, as provided in FIG. 1, shows that a carbon plus nitrogen content of about 0.75 percent and higher is required to satisfy the hardness requirement (35 R for valve steel applications. The

samples of TABLE 11 were solution treated at 2,150F for 1 hour and water quenched; each was aged at 1,400F for 16 hours.

The samples in identical bar form to those of TABLE I were evaluated for heat-treatment response at solution treating temperatures of 2,100, 2,l50 and 2,200F. With the high carbon plus nitrogen contents of each of the compositions, ranging from about 0.85 to 0.89 percent total carbon plus nitrogen, as may be seen from the hardness data presented in TABLE IIA, all of the samples obtained a solution treated and aged hardness in excess of 35 R which is desired for valve applications.

TABLE 11 RESULTS OF HARDNESS TESTS ON THE SILICON SERIES OF EXPERIMENTAL STEELS Hardness (R TABLE llA EFFECT OF SOLUTION TREATING TEMPERATURE ON HARDNESS Hardness (R,)

As previously mentioned, wear resistance is a highly important characteristic which valve materials must possess for use in low-lead or lead-free gasoline atmospheres. The test used to evaluate the wear resistance of the valve steels and valve steel-cast iron metal couples is fully described in Reference 1*. In this test, a stationary wear member is positioned at 90 to a rotating member that revolves at a speed of 670 rpm under a load of 7.5 pounds. Weight determinations are made on each wear member before and after the wear test to obtain data for calculating weight losses incurred during testing. These weight losses were then indicative of the wear resistance of the materials. A greater weight loss indicates less wear resistance. Wear tests were conducted on steels of TABLE 1 to demonstrate the effect of silicon and nickel on wear resistance. The effect of silicon on wear resistance of the valve steels (FIG. 2) and of the valve steel plus the cast iron couples (FIG. 3) shows that as the silicon content increases above 1.2 percent, the wear resistance decreases significantly. The effect of nickel on wear resistance (FIG. 4) shows that as the nickel content is increased to 1.8 percent, the total amount of wear increases markedly (TABLE III) even in steels that contain low silicon. This unexpected finding on the adverse effect of nickel on wear resistance clearly points out the criticality of minimizing nickel content so that no more than the residual nickel exists in the alloys in accordance with the present invention.

*6. Steven and .1. Pv Catlin A Controlled Dry Wear Test for High- Hardness Tool Steels," Journal of Materials, Vol. 1, 1966, page 293.

TABLE III WEAR TEST RESULTS Weight Loss (mg) Material Silicon Nickel Valve or Content Content Valve Cast Steel Bar No. Steel lron Cast lron 7l26 0.15 0.16 5.6 98.3 1039 71-29 1.20 0.12 4.9 71.8 76.7 71-27 2.21 0426 46.1 261.3 307.4 21-2N 0.12 1.83 68.1 344.8 412.9 21-4N 0.15 3177 74.4 45713 531.7

conditions used were those which caused a total deformation of 1 percent in hours for a conventional valve steel, namely 2l4N, at a typical valve service temperature of 1,350F as well as at a slightly higher valve service temperature of 1,500F. For testing, specimens from the bars of TABLES I and 1A were rough machined, heat treated and finish machined to conventional creep specimens having a 0.357 in. diameter by 2 in. long gauge section. Heat treatment consisted of solution treating at 2,150F for 1 hour, water quenching, and aging at 1,400F for 16 hours.

The stretch tests were conducted at 1,350F at a stress of 18,000 psi, and at 1,500F with a stress of 9,000 psi The results of the tests at both temperatures are presented in TABLE IV.

TABLE IV STRETCH TEST RESULTS Steel Test Test Time to Obtain Indicated or Bar Temp. Stress Total Elongation (hr) No. (F) (1000 psi) 05% 0.75% 1.0%

71-26 1350 18 68 124 168 71-27 1350 18 32 67 102 71-28 1350 18 30 93 198 71-29 1350 18 56 118 179 71-78 1350 18 32 79 126 71-79 1350 18 28 65 121 7l-80 1350 18 98 228 358 71-81 1350 18 40 104 179 71-82 1350 18 37 106 199 21-4N 1350 18 20 51 98 71-26 1500 9 64 132 200 71-27 1500 9 52 104 152 71-28 1500 9 163 375 530 7l-29 1500 9 86 118 71-78 1500 9 71 116 139 71-79 1500 9 144 199 233 71-80 1500 9 108 172 217 71-81 1500 9 177 319 410 71-82 1500 9 97 186 240 21-4N 1500 9 24 51 92 As may be seen from the data presented in TABLE IV, all of the steels possessed a stretch resistance comparable or superior to that of the control steel 2l4N. Steels which showed a notable improvement with regard to this particular property were the Bar No. 7128 steel from TABLE I, which steel contained 3 percent silicon as well as the high phosphorus steel Bar 7l-80 and the columbium-containing steel Bar 71-81 as well as the titanium strengthened steel Bar 7l82. As will be demonstrated hereinafter, however, the high silicon and phosphorus compositions although exhibiting superiority with regard to stretch resistance nevertheless exhibited poor hot workability and the titaniumcontaining steel, in addition, exhibited excessive oxidation or scaling.

TABLE V RESULTS OF 1400F OXIDATION TESTS ON SERIES I STEELS Steel Silicon Weight Gain after Indicated Time or Content at Temperature (mg/in?) Bar No. (71 20 40 6O 80 100 hr. hr. hr. hr. hr

The test specimens from which the TABLE V data were obtained were rough machined, heat treated at 2,150F for 1 hour, water quenched and aged for 16 hours at 1,400F and finish ground to size of 0.5 in. diameter by l-% in. length. The specimens were placed in porcelain crucibles and weight determinations were made. The specimens were then exposed in a static air atmosphere at a temperature of 1,400F for 20 hours, cooled to room temperature and then reweighed. This cycle was repeated five times to complete a IOO-hour test. Oxidation rate is expressed at the weight gain per unit area for the IOO-hour test. Again, the standard 2l-4N alloy was tested identically and used as a control. In addition to weighing the specimen after each cycle, a visual, qualitative examination was made of these characteristics by noting the presence and amount of scale in the bottom of the crucibles. The data of TABLE V show that all the alloys with mini mum chromium contents of 18 percent by weight have similar oxidation rates to the 2l-4N control. In addition, examination showed that the scaling characteristics were likewise similar. Results of oxidation tests conducted at 1,500F are given in TABLE VI for alloys ofTABLE I and in TABLE VII for alloys ofTABLE IA.

TABLE v1 RESULTS OF 1500F OXIDATION TESTS ON SERIES 1 STEELS Steel Silicon Weight Gain after Indicated Time or Content at Temperature (mg/in?) Bar No. 2O 40 6O hr. hr. hr. hr. hr.

TABLE VII RESULTS OF I500F OXIDATION TESTS ON SERIES 11 STEELS Steel Weight Gain after Indicated Time or Alloying at Temperature (mg/in?) Bar No. Element 20 40 6O 80 I 0 hr. hr. hr. hr.

71-78 15 Cr 5.6 7.5 9.1 10.0 11.2 71-79 20 Cr 65 9.2 11.1 12.4 13.4 7l-80 P 7.0 10.1 11.8 13.2 14.7 71-81 Cb 5.9 8.5 10.2 11.4 12.6 71-82 Ti 6.5 9.2 10.7 12.0 13.1 21-4N 4.6 8.2 8.9 9.0 9.3

These results show that, except for the low-silicon sample, e.g. Bar No. 71-26, the oxidation rate is comparable to the control 21-4N sample.

The hot-workability characteristics of all the steels were monitored during forging of the ingots. This qualitative examination showed that the aluminumcontaining steels, e.g. Bar Nos. 71-31 and 71-32, which had more than about 1 percent aluminum and the high phosphorus sample, e.g. Bar No. 71-80, were prone to extensive surface cracking and corner cracking during this forging operation. In view of this determination with regard to hot workability tensile strength determinations were made to provide a quantitative determination of the effect of varying the total carbon plus nitrogen, with various chromium and silicon contents with regard to hot workability, the results of which are presented in TABLES VIII and IX.

TABLE VIII RAPID-STRAIN-RATE TENSILE STRENGTHS OF EXPERIMENTAL STEELS AND 2l-4N Tensile Strength (1000 psi) Test 71-27 71-78 7149 Temp. 71-25 71-29 2% Si) 71-28 (2% Si) (2% s1) (F) (15% so (1% s1) (18% Cr) (3% s11 (15% Cr) 20% Cr) 21-419 TABLE 1x Reduction in Area (/r) Test 71-27 71-78 71-79 Temp. 71-26 71-29 12% 51) 71-28 2% s1) 2% s1) (F) 1.15% 51) 1% s1 (13% Cr) 13% Si) (15% c1) Cr) 21-4N Specimens of 0.252 in. diameter by 1 in. gauge length in the hot-rolled condition were direct resistance rate of about 550 in./minute. A rapid response recorder was used to obtain the breaking loads for calculation of the tensile strength. With respect to TABLE VIII, comparison of the strength values at 2,-100F shows that the steels have slightly higher strength than the 21-4N control and, therefore, may be regarded as more resistant to deformation. However, a plot of the carbon plus nitrogen content versus the 2,100F tensile strength as presented in FIG. 5 shows that the strength increases with the increase in carbon plus nitrogen content thus indicating the significance of the total combination of these elements.

With respect to TABLE IX, and the curve of FIG. 6 based thereon, the data show that for the 2 percent silicon steels containing 15 percent, 18 percent and 20 percent chromium, the 18 percent chromium steel offers a ductility advantage over the 15 percent and 20 percent chromium steels. The data have also been plotted in FIG. 7 to show that the lower ductility of the 15 and 20 percent chromium steels, however, is attributable to the higher carbon plus nitrogen content. The hot workability curves for the 18 percent chromium steels containing 0.2, 1, 2 and 3 percent silicon are shown in FIG. 8. These curves show that the elevated tensile ductility decreases sharply with increases in silicon. The effect of silicon in this regard is best shown by comparing the hot ductility curves of the l and 2 percent silicon steels, which steels are virtually otherwise identical in composition. It may be noted that the hot ductility of the 1 percent silicon steel is excellent even at 2,200F; whereas, the hot ductility of the 2 percent silicon steeldecreases significantly above 2,100F. This establishes the upper limit of silicon in the alloy for purposes of valve manufacture at 1.5 percent.

We claim:

1. An internal combustion engine valve made from an essentially nickel-free austenitic stainless steel consisting essentially of, in weight percent, 0.25 to 0.55 carbon, 0.05 to 0.45 nitrogen, with carbon plus nitrogen within the range of 0.65 to 0.95, 10 to 14 manganese, 0.50 to 1.5 silicon, 15 to 20 chromium, 1 max. aluminum, an effective amount up to 1.0 of at least one element selected from the group consisting of columbium, tantalum, titanium, zirconium and hafnium, 0.0008 to 0.005 boron and balance iron.

2. The steel of claim 1 wherein chromium is present within the range of 17 to 19 percent by weight.

3. The steel of claim 1 wherein columbium is present Within the range of 0.01 to 0.5 percent by weight.

4. The steel of claim 1 wherein boron is present within the range of 0.0008 to 0.0015 percent by weight.

5. An internal combustion engine valve made from an austenitic stainless steel consisting essentially of, in weight percent, 0.25 to 0.55 carbon, 0.05 to 0.45 nitrogen, with carbon plus nitrogen within the range of 0.65 to 0.95, 10 to 14 manganese, 0.50 to 1.5 silicon, 17 to 19 chromium, 1 max. aluminum, an effective amount up to 1.0 of at least one element selected from the group consisting of columbium, tantalum, titanium, zirconium and hafnium, 0.0008 to 0.0015 boron and balance iron. 

2. The steel of claim 1 wherein chromium is present within the range of 17 to 19 percent by weight.
 3. The steel of claim 1 wherein columbium is present within the range of 0.01 to 0.5 percent by weight.
 4. The steel of claim 1 wherein boron is present within the range of 0.0008 to 0.0015 percent by weight.
 5. An internal combustion engine valve made from an austenitic stainless steel consisting essentially of, in weight percent, 0.25 to 0.55 carbon, 0.05 to 0.45 nitrogen, with carbon plus nitrogen within the range of 0.65 to 0.95, 10 to 14 manganese, 0.50 to 1.5 silicon, 17 to 19 chromium, 1 max. aluminum, an effective amount up to 1.0 of at least one element selected from the group consisting of columbium, tantalum, titanium, zirconium and hafnium, 0.0008 to 0.0015 boron and balance iron. 